The present invention relates to a process for producing a thin film semiconductor device having a semiconductor thin film. More specifically, the present invention relates to a thin film semiconductor device useful for use in such applications as image display, a production process therefor, and an information display (image display device).
The conventional method of crystallizing an amorphous silicon thin film through scanning of a pulsed laser will be described with reference to FIG. 10. FIG. 10 is a diagram showing the most common conventional crystallization method which uses an excimer pulsed laser. In this method, a laser beam 105 which is a linear excimer laser having a width L on the order of several millimeters on a substrate 100 is irradiated onto an amorphous silicon film 102 deposited over the substrate 100 through a bottom film 101. Then, the irradiating position of the laser beam 105 is shifted for every one to several pulses, thereby crystallizing the entire substrate. In this conventional method, when the laser beam is irradiated, crystal nuclei are generated in a random fashion. Further, under an ordinary laser annealing condition, an average distance between the generated crystal nuclei is 0.5 microns or less. Accordingly, the grain size of crystals in a resulting polycrystalline silicon film 103 is 0.5 microns or less, and the size of the grains is not uniform.
In WO-9745827, the following method is disclosed. The width L of the laser beam 105 illustrated in FIG. 10 is reduced to 0.5 microns or less. Then, with the position of the laser beam 105 having this shape being shifted at each interval of 0.5 microns or less, the irradiation of the pulsed laser is performed. Then, the resulting crystals grow in one orientation, starting from an initially generated crystal grain as a seed crystal. The “one orientation” herein refers to a direction perpendicular to a lateral direction or the thickness of the film formed of grown crystals.
JP-A-2000-68520 discloses the following method. In this method, as a bottom film for an amorphous silicon thin film, underlying layer films having different thermal conductivity are arranged in the form of stripes. With this arrangement, when the melting crystallization is performed by the irradiation of the excimer laser, the generating positions of crystal nuclei are controlled. A high-temperature region in contact with the underlying layer film with low thermal conductivity causes few defects in the silicon region, while a low-temperature region in contact with the underlying layer film with high thermal conductivity causes a lot of defects in the silicon region.
In the conventional methods described above, the time required for crystal growth is 100 nanoseconds or shorter. Thus, the size of the resulting crystal grains is 1 micron or less and exhibited a wide range of variation. Further, the orientations of the crystal grains are random, the density of defects is large, and the degree of surface film roughness is great. Thus, it is impossible to cause polycrystalline silicon having a large grain size to grow, or to accurately control the grain size or the positions of grain boundaries. This leads to random inclusion of the grain boundaries within a channel. Hence, it is difficult to improve device characteristics, reliability and device uniformity in a TFT array.
In the method disclosed in WO-9745827, the laser beam must be converged to a spot of several microns or less in diameter. Thus, a laser energy loss occurs, and an optical system for laser irradiation becomes complicated. Further, since the moving distance of the laser beam between pulses is several microns or less, it takes much time to crystallize the entire substrate. For this reason, achievement of a high throughput and a low cost is difficult. In particular, this method is not adaptable to a large-area substrate. Further, minute-distance travel is subject to the influence of vibration, which caused a problem in terms of yields. In the resulting semiconductor thin film, the crystal defects are induced in a direction corresponding to the scanning direction of the substrate. The orientations of the resulting grain boundaries can not be controlled, and it is difficult to improve device characteristics and uniformity in a TFT array. Further, it is impossible to make the channel free of grain boundaries.
On the other hand, in the method disclosed in JP-A-2000-68520, the positions of crystal nuclei can be partly controlled. However, it is difficult to ensure an area sufficient for disposing a thin film semiconductor device, so that improvement in device performance is impossible.